NAME

DBIx::Class::Manual::FAQ - Frequently Asked Questions (in theory)

DESCRIPTION

This document is intended as an anti-map of the documentation. If you know what you want to do, but not how to do it in DBIx::Class, then look here. It does not contain much code or examples, it just gives explanations and pointers to the correct pieces of documentation to read.

FAQs

How Do I:

Getting started

.. create a database to use?

First, choose a database. For testing/experimenting, we recommend DBD::SQLite, which is a self-contained small database (i.e. all you need to do is to install DBD::SQLite from CPAN, and it works).

Next, spend some time defining which data you need to store, and how it relates to the other data you have. For some help on normalisation, go to http://b62.tripod.com/doc/dbbase.htm.

Now, decide whether you want to have the database itself be the definitive source of information about the data layout, or your DBIx::Class schema. If it's the former, look up the documentation for your database, eg. http://sqlite.org/lang_createtable.html, on how to create tables, and start creating them. For a nice universal interface to your database, you can try DBI::Shell. If you decided on the latter choice, read the FAQ on setting up your classes manually, and the one on creating tables from your schema.

Once you have created all the appropriate table/source classes, and an overall Schema class, you can start using them in an application. To do this, you need to create a central Schema object, which is used to access all the data in the various tables. See "connect" in DBIx::Class::Schema for details. The actual connection does not happen until you actually request data, so don't be alarmed if the error from incorrect connection details happens a lot later.

.. use DBIx::Class across multiple databases?

If your database server allows you to run queries across multiple databases at once, then so can DBIx::Class. All you need to do is make sure you write the database name as part of the table call. Eg:

This is called a has_many relationship on the one side, and a belongs_to relationship on the many side. Currently these need to be set up individually on each side. See DBIx::Class::Relationship for details.

.. define a foreign key relationship where the key field may contain NULL?

Just create a belongs_to relationship, as above. If the column is NULL then the inflation to the foreign object will not happen. This has a side effect of not always fetching all the relevant data, if you use a nullable foreign-key relationship in a JOIN, then you probably want to set the join_type to left.

.. define a relationship where the key consists of more than one column?

Instead of supplying a single column name, all relationship types also allow you to supply a hashref containing the condition across which the tables are to be joined. The condition may contain as many fields as you like. See DBIx::Class::Relationship::Base.

.. define a relationship bridge across an intermediate table? (many-to-many)

By default, DBIx::Class cascades deletes and updates across has_many and might_have relationships. You can disable this behaviour on a per-relationship basis by supplying cascade_delete => 0 in the relationship attributes.

The cascaded operations are performed after the requested delete or update, so if your database has a constraint on the relationship, it will have deleted/updated the related records or raised an exception before DBIx::Class gets to perform the cascaded operation.

to search, will probably not do what you expect. It will quote the text "now()", instead of trying to call the function. To provide literal, unquoted text you need to pass in a scalar reference, like so:

The first argument to search is a hashref of accessor names and values to filter them by, for example:

->search({'created_time' => { '>=', '2006-06-01 00:00:00' } })

Note that to use a function here you need to make it a scalar reference:

->search({'created_time' => { '>=', \'yesterday()' } })

.. search in several tables simultaneously?

To search in two related tables, you first need to set up appropriate relationships between their respective classes. When searching you then supply the name of the relationship to the join attribute in your search, for example when searching in the Books table for all the books by the author "Fred Bloggs":

->search({'authors.name' => 'Fred Bloggs'}, { join => 'authors' })

The type of join created in your SQL depends on the type of relationship between the two tables, see DBIx::Class::Relationship for the join used by each relationship.

.. create joins with conditions other than column equality?

Currently, DBIx::Class can only create join conditions using equality, so you're probably better off creating a view in your database, and using that as your source. A view is a stored SQL query, which can be accessed similarly to a table, see your database documentation for details.

.. search with an SQL function on the left hand side?

To use an SQL function on the left hand side of a comparison you currently need to resort to literal SQL:

which (if supported by the database) will use LIMIT/OFFSET to hint to the database that we really only need one row. This can result in a significant speed improvement. The method using "single" in DBIx::Class::ResultSet mentioned in the cookbook can do the same if you pass a rows attribute to the search.

Discarding changes and refreshing from storage are two sides of the same coin. When you want to discard your local changes, just re-fetch the row from storage. When you want to get a new, fresh copy of the row, just re-fetch the row from storage. "discard_changes" in DBIx::Class::Row does just that by re-fetching the row from storage using the row's primary key.

.. fetch my data a "page" at a time?

Pass the rows and page attributes to your search, eg:

->search({}, { rows => 10, page => 1});

.. get a count of all rows even when paging?

Call pager on the paged resultset, it will return a Data::Page object. Calling total_entries on the pager will return the correct total.

count on the resultset will only return the total number in the page.

Inserting and updating data

.. insert a row with an auto incrementing primary key?

This happens automatically. After creating a result object, the primary key value created by your database can be fetched by calling id (or the access of your primary key column) on the object.

.. insert a row with a primary key that uses a sequence?

You need to create a trigger in your database that updates your primary key field from the sequence. To help PK::Auto find the next key value, you can tell it the name of the sequence in the column_info supplied with add_columns.

This technique is an easy way to store supplemental unstructured data in a table. Be careful not to overuse this capability, however. If you find yourself depending more and more on some data within the inflated column, then it may be time to factor that data out.

Custom methods in Result classes

You can add custom methods that do arbitrary things, even to unrelated tables. For example, to provide a $book->foo() method which searches the cd table, you'd could add this to Book.pm:

With either of these methods the resulting use of the accessor would be

my $result;
# assume that somewhere in here $result will get assigned to a MyTable row
$result->non_column_data('some string'); # would set the non_column_data accessor
# some other stuff happens here
$result->update(); # would not inline the non_column_data accessor into the update

How do I use DBIx::Class objects in my TT templates?

Like normal objects, mostly. However you need to watch out for TT calling methods in list context. When calling relationship accessors you will not get resultsets, but a list of all the related objects.

For more info see DBIx::Class::Storage for details of how to turn on debugging in the environment, pass your own filehandle to save debug to, or create your own callback.

Why didn't my search run any SQL?

DBIx::Class runs the actual SQL statement as late as possible, thus if you create a resultset using search in scalar context, no query is executed. You can create further resultset refinements by calling search again or relationship accessors. The SQL query is only run when you ask the resultset for an actual result object.

How do I deal with tables that lack a primary key?

If your table lacks a primary key, DBIx::Class can't work out which row it should operate on, for example to delete or update. However, a UNIQUE constraint on one or more columns allows DBIx::Class to uniquely identify the row, so you can tell DBIx::Class::ResultSource these columns act as a primary key, even if they don't from the database's point of view:

If you just want to override the original method, and don't care about the data from the original accessor, then you have two options. Either use Method::Signatures::Simple that does most of the work for you, or do it the "dirty way".

Troubleshooting

Help, I can't connect to postgresql!

If you get an error such as:

DBI connect('dbname=dbic','user',...) failed: could not connect to server:
No such file or directory Is the server running locally and accepting
connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?

Likely you have/had two copies of postgresql installed simultaneously, the second one will use a default port of 5433, while DBD::Pg is compiled with a default port of 5432.